Prior art in the lacquering of motor vehicles or parts thereof is initially the production of the body or the body parts. For this, correspondingly shaped sheet metal parts are first stamped out and shaped by the deep-drawing process using drawing lubricant or oil, and are then in general assembled by welding and subsequently subjected to expensive cleaning. A lacquer coat is then applied by electro-diplacquering and, if desired, one or, in the case of automobile bodies in particular, more further coats of lacquer are applied.
There is the desire to discover a simpler production process which, in particular, avoids the cleaning steps and enables the electro-diplacquering to be replaced.
European Patent Application EP-A-0 603 772 discloses a process for the production of vehicle parts from prelacquered sheets. The sheets, which have been prelacquered with a primer, are cut into sections, and these are deep-drawn and coated with a powder lacquer as the top lacquering. The lacquers used to produce the primer and the top lacquering are not described in more detail.
European Patent Application EP-A-0 438 773 discloses a process for the production of motor vehicle bodies in which sheets which have been precoated with a lacquering of at least two coats are deep-drawn and then assembled. An acrylate lacquer is employed as the primer and a polyurethane lacquer is employed as the top lacquer. The compositions of these lacquers is not explained in more detail.
WO 89/11497 describes a coating composition which can be applied by the coil coating process and is suitable for applying one-coat lacquerings, which can be subjected to deep-drawing, to sheet metal. After correspondingly shaped sheets have been stamped out, the parts lacquered with one coat in this way can be shaped by the deep-drawing process without the lacquer coat showing damage. The coating compositions from WO 89/11497 comprise a polyester-polyol with a controlled degree of branching and a weight-average molecular weight of 4,000 to 40,000 and a hydroxyl number of 10 to 100 mg KOH/g as the binder, which is crosslinked via free or blocked polyisocyanate.
The coatings produced by the coil coating process using the coating compositions from WO 89/11497 are in need of improvement in respect of their deep-drawing properties. This has been found in the case of coil coating application to both sides, where the coating layer applied to the later inside of the shaped part does not withstand the shaping process, which has a particularly extreme effect there. This manifests itself, for example, in the simulation of the deep-drawing operation in which a square dish with rounded corners is produced from a sheet coated on both sides by the coil coating process. The coating in the region of the corners on the inside of the dish shows cracks or even flaking, especially if sheets having a thickness in the range from 0.7 to 1.3 mm are employed, such as are usual in automobile coach construction. Furthermore, the deep-drawn one-coat lacquerings applied by the coil coating process do not adhere to the substrate if further coats of lacquer, which are usual in car series lacquering, are applied, such as, for example, a filler, base lacquer and clear lacquer coat. It has been found that the coil coating primer coat becomes detached from the metallic substrate, together with the subsequent lacquer coats, especially in the regions of severe shaping. For example, detachment of the multi-coat lacquering manifests itself in the region of the corners and edges on the outside of the square dish, often already after stoving of the filler coat.